So if people are finding the subclasses restrictive compared to, say, 3e, let's compare them to their 3e equivalents, shall we?
Barbarians, monks, rangers, sorcerers, paladins: The 3e version of each of these classes would fall entirely into a single one of their 5e version's subclass. Totem warrior, ninja, elementalist, beastmaster, wild mage, avenger, and green knight are all concepts you couldn't achieve in 3e without multiclassing or prestige classes. So really the 5e versions are using subclasses to expand these archetypes, not restrict them.
Wizards, clerics: these classes are restricted by school and domain in 3e too, and in the wizard's case those restrictions are far more onerous in 3e.
Warlock: not in 3e core, but the 4e version had different pacts, and like with clerics and sorcerers, the pact seems so essential to the class that you really need it from level 1.
Fighters: This is a slightly weird case, since the 3e fighter was more flexible than the Champion but didn't have maneuvers like the Battlemaster.
Rogues, bards: These are the only two classes, IMO, that are "sliced up" in 5e but were whole in 3e.